There has been a long felt desire for a modulated pressure relief valve on a vessel for venting fluid when the pressure exceeds a desired dynamic set-point or threshold. A particular desire is for a modulated pressure relief valve with a threshold that could vary according to process requirements and its upper limit could serve to avoid blowing a safety relief device such as an expendable rupture disc or pop-off safety relief valve both of which are typically designed for infrequent use. It is desirable to communicate the desired pressure set-point to the relief valve via a reference pressure signal (typically air) that is equal to the desired vessel pressure.
Furthermore, it is desirable to have a modulating pressure relief valve which is very simple, with cleanable parts and with no narrow passageways that can become plugged with debris or frozen product. Such a device would be useful in many applications including the food, beverage, gelatin, and polymer industries, where the product can become frozen in critical passages, and where all crevices must be exposed to the rinsing and cleaning processes.
Typical safety relief valves, such as those exemplified in U.S. Pat. No. 6,095,183, include a valve member biased by a spring into engagement with a seat. If product pressure exceeds a predetermined level, the force against the valve exceeds the biasing force of the spring causing the valve to lift from the seat to vent product pressure.
Typical diaphragm relief valves, such as those exemplified in U.S. Pat. No. 5,944,050, do present simple, cleanable surfaces yet they clearly do not meet the dynamic set-point requirement desired in the art. Furthermore, the inaccuracies of the spring compression typically result in wide variations in relieving pressures, often greater than 10%, and the variations change over time.
Dynamically modulated relief valves are provided that use a reference or pilot signal as exemplified in U.S. Pat. No. 6,318,406. These typically involve complex spring and seal mechanisms. Besides the inherent robustness issues with the complex mechanisms, they are clearly not acceptable for use in processes with debris, freezeable product, or requiring cleanability.
Typical relief valves are quite insensitive to slight differences in pressure, and typically have large cracking pressure biases. A highly sensitive relief valve would help maintain a constant vessel pressure during changes in the upstream process environment which is a feature that is very useful in many industrial steady-state applications, including constant-flow applications.
Further, there has been a long felt desire for a relief valve that could balance vessel pressure exactly to a reference, or pilot, pressure. This would facilitate a complete vessel pressure control system when used in conjunction with a simple instrument pressure sender and a, preferably no-loss, check valve.
Typically available pilot actuated relief valves cannot control vessel pressure to the exact pressure of the reference signal.
Many fluid systems also exhibit pulsations, which are typically caused by mechanical sources such as pump movements or valve closures. An additional type of pulsation is found with certain sensitive fluid control devices, which can exhibit undesirable chatter (or pulsations) when used with incompressible fluids. An example of this phenomenon occurs when a positive pressure wave travels upstream and strikes the outlet of the regulator, whereby said positive pressure wave temporarily imbalances the membrane and causes a positive flow bias to emanate from the device. Such a positive flow bias becomes a positive pressure wave traveling back downstream. This response manifests itself as an echo, allowing resonance to become established in the fluid conduit downstream of the fluid control device.
It is desirable to eliminate such pulsations in such fluid systems. Furthermore, it is desirable to inhibit said chatter in a way that it permanent and robust, with no maintenance required. Furthermore, it is desirable to inhibit said chatter in a way that is hygienic and allows for easy cleaning of the fluid system. Furthermore, it is desirable to inhibit said chatter in a way is economical, easy to incorporate into the fluid system design, does not require a separately sealed component, and cannot obstruct the flow through the system.
It is known in the art that altering the compressibility of a fluid system can control the resonant chatter in said system. Said compressibility is traditionally established by incorporating a pulsation dampener, which typically involves the addition of a gas pocket into the system. Said gas pocket may be in direct contact with the fluid, or may be separated by a flexible membrane. The gas pocket typically requires the addition of a separate fluid component, with its costs, space requirements, maintenance requirements, costs, connections, and potential for leaks. Furthermore, gas tends to become absorbed over time into liquids where direct contact exists, requiring some form of replenishment. Gas can also leak out slowly through flexible separation membranes over time as well due to membrane permeability or microscopic leaks. Another pulsation dampening method existing in the art is the use of tubing with flexible walls. However, the use of flexible conduit material requires additional sealing points, and is limited by the pressure rating of said flexible tubing.